The present invention relates to purification of liquid lithium for removal of unwanted quantities of dissolved nitrogen or aluminum. According to the disclosure, the impurities are precipitated from the lithium as aluminum nitride.
Liquid lithium has been proposed as the working coolant in fusion reactors and related test facilities. It is expected that stainless steel will be the primary lithium containment material. This disclosure is directed to the resulting corrosion problems that are anticipated in such facilities. While even pure lithium exhibits some corrosive effects on stainless steel, these effects are greatly accentuated and changed in nature by the presence of certain lithium impurities, particularly dissolved quantities of carbon and nitrogen. Dissolved nitrogen is of the greatest concern because of its high solubility in lithium and the ease of contamination of the lithium by contact with surrounding air.
The effects of nitrogen on corrosion of stainless steel by lithium has been previously documented by researchers. They have found that corrosion effects are influenced by the nitrogen concentration. High nitrogen levels typically lead to general dissolution and intergranular penetration in the stainless steel structure. It appears that the primary attack is via chromium, which forms a ternary nitride, Li.sub.9 CrN.sub.5. To maintain lithium at an acceptable corrosion level, it appears essential to keep its dissolved nitrogen content below about 100 ppm.
Aluminum can also be a major impurity in commercial grade lithium. It carries over from the lithium ore. Dissolved quantities of aluminum in liquid lithium are not known to cause corrosion in stainless steel or other potential containment metals. However, in the presence of dissolved aluminum, subsequent contamination of the liquid lithium by nitrogen results in the formation of a solid phase (aluminum nitride) which might settle as a sludge or deposit. This solid material can inhibit lithium flow characteristics or plug small flow paths. It also will adversely affect heat transfer capabilities of the liquid lithium.
The present method comprises a chemical system for removing aluminum or nitrogen from liquid lithium prior to or during its use.
Prior U.S. Pat. No. 1,559,342 to Mattice describes attempts to purify molten metal by the use of aluminum. It does not identify the impurities removed by the aluminum, nor does it describe the specific molten metal that is being purified. It relates to an improved technique of adding lime, borax and aluminum. The reaction forms a slag which can be easily separated from the molten metal.
Prior U.S. Pat. Nos. 1,802,693 to Anderson, and 4,003,560 to Carbonnel each disclose methods for removing impurities from a metal by bubbling nitrogen through the molten metal. Both are primarily directed to purification of molten aluminum. The nitrogen apparently does not react with the aluminum, but only with the unidentified impurities which are removed from it.
U.S. Pat. No. 2,997,289 to Baker et al is of background interest in that it describes an apparatus for producing lithium metal and separating the lithium from barium.